Reverted to calling read/write uint256

packages/contracts/src/contracts/current/protocol/AssetProxyDispatcher/proxies/ERC721Proxy.sol

@@ -67,7 +67,7 @@ contract ERC721Proxy is
     {
         require(assetMetadata.length == 53);
         tokenAddress = readAddress(assetMetadata, 1);
-        tokenId = uint256(readBytes32(assetMetadata, 21));
+        tokenId = readUint256(assetMetadata, 21);
         return (tokenAddress, tokenId);
     }
 
@@ -90,7 +90,7 @@ contract ERC721Proxy is
         assetMetadata = new bytes(53);
         assetMetadata[0] = byte(assetProxyId);
         writeAddress(assetMetadata, 1, tokenAddress);
-        writeBytes32(assetMetadata, 21, bytes32(tokenId));
+        writeUint256(assetMetadata, 21, tokenId);
         return assetMetadata;
     }
 }

packages/contracts/src/contracts/current/utils/LibBytes/LibBytes.sol

@@ -123,4 +123,30 @@ contract LibBytes {
             mstore(add(b, index), input)
         }
     }
+
+    /// @dev Reads a uint256 value from a position in a byte array.
+    /// @param b Byte array containing a uint256 value.
+    /// @param index Index in byte array of uint256 value.
+    /// @return uint256 value from byte array.
+    function readUint256(
+        bytes memory b,
+        uint256 index)
+        public pure
+        returns (uint256 result)
+    {
+        return uint256(readBytes32(b, index));
+    }
+
+    /// @dev Writes a uint256 into a specific position in a byte array.
+    /// @param b Byte array to insert <input> into.
+    /// @param index Index in byte array of <input>.
+    /// @param input uint256 to put into byte array.
+    function writeUint256(
+        bytes memory b,
+        uint256 index,
+        uint256 input)
+        public pure
+    {
+        writeBytes32(b, index, bytes32(input));
+    }
 }